What is Plankton?

One night, we’re reading a book about whales. It was the prize in a kid’s meal from Chik-Fil-A, and chock full of pictures of whales and dolphins, and he loved it. So we get to the page about the Blue Whale, and he asks what it eats. “Plankton,” I tell him.

He thinks about that for a moment. “What’s plankton?”

I think about that for a moment. “Little tiny plants,” I say. “And tiny shrimp.”

“Ew,” he says, wrinkling his nose.

So there you have it. Whales eat gross stuff.



It turns out that I was wrong. Plankton is not a type of creature, but a lifestyle – it’s defined as “the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily comprising microscopic algae and protozoa.” This is contrasted with nekton, which is “the aggregate of actively swimming aquatic organisms in a body of water, able to move independently of water currents.” The word comes from the German word Plankton from the Greek word plankton, meaning “wandering, drifting”. That words derives from the proto-Indo-European word *plak-, “to strike, hit”.

There are two different ways to divide plankton:  either by the kingdom the plankton falls into (phytoplankton, zooplankton, and bacterioplankton), or by whether or not the organisms are permanently plankton (holoplankton or meroplankton).


Phytoplankton, also known as microalgae, is “little tiny plants”.  They’re chlorophyll-containing organisms that float in the upper reaches of the ocean where sunlight can penetrate – generally the euphotic region, which is about 200 meters deep.  For the most part, phytoplankton is found along the coasts, in the upper northern and southern latitudes, and along the equator.

There are two magor groups of phytoplankton, dinoflagellates and diatoms.  According to NOAA, “Dinoflagellates use a whip-like tail, or flagella, to move through the water and their bodies are covered with complex shells. Diatoms also have shells, but they are made of a different substance and their structure is rigid and made of interlocking parts. Diatoms do not rely on flagella to move through the water and instead rely on ocean currents to travel through the water.”


Zooplankton include the “tiny shrimp”, just like I told my son.  However, there is a whole lot more to them than “tiny shrimp”.  They include shrimp, worms, water fleas, isopods, tunicates, the larval form of larger organisms, and any other sort of poorly-swimming oceanic ocean life.  This includes jellyfish.

Zooplankton are further classified by size:

Size categories include:  picoplankton that measure less than 2 micrometers, nanoplankton measure between 2-20 micrometers, microplankton measure between 20-200 micrometers, mesoplankton measure between 0.2-20 millimeters, macroplankton measure between 20-200 millimeters, and the megaplankton, which measure over 200 millimeters (almost 8 inches).


Many sources classify bacteroplankton as part of zooplankton, but others classify them as seperate because bacteria are in one of two kingdoms (Archaebacteria or Eubacteria) that is entirely separate from Animalia.  In brief, bacteroplankton are ocean-living bacteria, and there are a lot of them.  Estimates are that the ocean contains 3.1 x 1028 of them, which is a number I won’t type out because it is long and tedious to do so.

Holoplankton and Meroplankton

These two classifications merely describe whether the planktonic organism remains planktonic throughout its entire lifecycle.  Holoplankton are larval organisms, eventually maturing into nektonic creatures, while meroplankton are organisms that remain planktonic throughout their entire lifecycle.

So, next time you go… have fun at the beach.